I think I finally figured out you can only delete your posting if it's the last one in the thread. Maybe this was obvious to others, but it was a mystery to me. You can of course edit a posting and delete all but one character.

What is the number of posts a new user must make before being able to attach a file? Or is it some other criterion besides number of postings? It would be useful to know this when trying to solve a new person's problem.